The present invention relates to a power converter apparatus being formed with a DC voltage source which is powered by an AC power supply, and being adapted to a load device for the DC voltage source.
The combination of a pulse-width-modulation controlled (PWM) inverter and an induction motor, the combination of a DC chopper and a DC motor, etc., are conventionally used as a load device powered by a DC voltage source. No practical problem arises where a battery is used as the DC voltage source. However, when an AD/DC power converter is used for obtaining a DC voltage from an AC power line, the AC power line is subjected to inactive power and/or higher harmonics caused by the converter operation. This is a significant problem, and has recently come to the forefront.
To solve the above problem, it is proposed that a PWM converter be used as an AC/DC power converter being arranged between an AC power line and a DC voltage souce (capacitor) (cf. Japanese Patent Disclosure No. 59-61475).
FIG. 1 shows a prior art power converter apparatus in which a large power capacity is achieved by a parallel connection of PWM converters.
In FIG. 1, the reference symbol SUP denotes a single-phase AC power supply. The reference symbol TR denotes a power transformer. The reference symbols Ls1 and Ls2 respectively denote AC reactors. The reference symbols CONV1 and CONV2 denote PWM converters. The reference symbol Cd denotes the filtering capacitor for a DC voltage source. The reference symbol INV denotes a PWM inverter which converts a DC voltage into a voltage-variable and frequency-variable, 3-phase AC voltage. The reference symbol IM denotes a 3-phase induction motor. PWM inverter INV and induction motor IM constitute a load device for DC voltage source Cd. AC reactors Ls1 and Ls2 serve to achieve the current balancing for respective converters CONV1 and CONV2, and also serve to suppress pulsate variations of input currents Is1 and Is2.
The control operation of the load device will be as follows.
The rotation speed N of induction motor IM is detected by a speed detector PG. The detected value N of the rotation speed is compared with a given speed instruction N*. Rotation speed N is controlled by a speed control circuit SPC, so that the detected speed value N becomes substantially equal to the value of speed instruction N*.
An output signal IL* from speed control circuit SPC defines the instruction value for 3-phase currents IL actually supplied to induction motor IM. The value of actual currents IL is compared with current instruction IL*. Then, currents IL are controlled by a load current control circuit ALC, so that the value of actual currents IL becomes substantially equal to the value of current instruction IL*. An inverter-side PWM control circuit PWM1 controls inverter INV in accordance with an output signal from load current control circuit ALC.
On the other hand, converters CONV1 and CONV2 control a current Is supplied from power supply SUP, so that the value of the DC voltage Vd appearing across filtering capacitor Cd becomes substantially constant. In other words, the detected value of DC voltage Vd is compared with a DC voltage instruction Vd*, and control by means of a voltage control circuit AVC is effected on current Is so that the value of voltage Vd becomes substantially equal to the value of instruction Vd*. Namely, an output signal Is* from voltage control circuit AVC defines the instruction value for current Is supplied from power supply SUP. The detected value of input current Is is compared with current instruction Is*, and current Is is controlled by an input current control circuit ASC so that the value of current Is becomes substantially equal to the value of instruction Is*. A converter-side PWM control circuit PWMc controls converters CONV1 and CONV2 in accordance with an output signal from input current control circuit ASC.
In the above prior art power converter apparatus, a current Is supplied from the AC power supply is controlled so that the value of a volage Vd appearing across DC voltage source Cd becomes substantially constant. Such a prior art apparatus has the following features:
(1) four quadrant operation, as well as regenerating operation, are both available according to the power required by the load device.
(2) The phase of input current Is is controlled to be matched with the phase of a power supply voltage Vs, so that the input power factor is kept at "1".
(3) The waveform of input current Is is controlled to be sinusoidal, so that higher harmonic components of current Is can be effectively reduced.
The above prior art power converter apparatus encounters the following disadvantages.
(1) A PWM converter performs a switching operation with a modulation frequency of several kHz. For this reason, GTOs (gate turn-off thyristors) are often required. Generally speaking, the maximum ratings of a GTO with respect to the withstanding voltage and the current capacity are lower than those of a general thyristor. From this, it is difficult to obtain a high-power converter using GTOs.
To increase the power capacity, a parallel connection of converters, as shown in FIG. 1, is conventionally employed. According to such a converter configuration, a high-power apparatus requires a large number of GTOs. This enlarges the size or dimensions of the apparatus, and increases the manufacturing cost thereof.
The prior art power converter apparatus also encounters the following disadvantages.
(2) GTOs are often used to constitute the PWM converter. Such GTOs inevitably involve ineffective operating periods, i.e., a minimum on period and a minimum off period. From this, once a turn-on signal is generated, the generation of a turn-off signal is inhibited for a period of 100 to 300 .mu.s. Similarly, once the turn-off signal is generated, the generation of the turn-on signal is also inhibited for a period of 100 to 300 .mu.s. Thus, the converting efficiency becomes worse as the carrier frequency of the PWM control (the switching frequency of switching elements) becomes high. This requires the reduction in the power supply voltage (secondary voltage of the transformer). If the output power capacity is fixed, the reduction in the secondary voltage of the transformer causes the increase of the input current. Then, the current capacity of the switching elements must be correspondingly increased.
Self-turn-off devices, such as GTOs, having a withstanding voltage of 4500 V and having a shut-off current of 2000 A are currently manufactured. Such high-rated GTOs are used primarily to constitute high-power PWM converters. In practice, however, because of said ineffective operating periods, the possible maximum carrier frequency of the PWM control operation can be at most 500 Hz to 1 kHz.
(3) When a plurality of converters are parallel connected to enhance the power capacity, the same number of AC reactors as the number of parallel converters must be provided at the secondary circuit of the power transformer, thereby achieving the input current balancing. Respective AC reactors serve to suppress higher harmonic components involved in the AC output voltage. Such higher harmonic components are caused by the switching operation of the PWM converter. Respective AC reactors also serve to remove pulsate variations in the input current supplied from the power supply. Since, as mentioned before, the switching frequency (carrier frequency) of the respective converters is at most 500 Hz to 1 kHz, a rather large inductance is required for each converter reactor.
(4) According to the prior art apparatus, large capacity AC reactors must be provided for respective converters. From this, it is difficult to obtain a compact and light-weight apparatus. In many cases, when the space for placing a converter apparatus is limited, the prior art apparatus cannot be employed.